I had cataract surgery in Sep 2018. I now have an under eye bag under one eye. What could cause this?

Thank you for sharing your question. Some patients notice their lower eyelids more after cataract surgery. Others coincidentally simply start getting bags in their lower eyelids following cataract surgery with no correlation to the surgery. If your puffiness bothers you enough, you can consider having blepharoplasty performed. I hope this helps!
Good luck,

It is not usual to develop eye bags or puffy eyes after cataract surgery. However, the inflammation associated with the surgery and post op recovery could lead to accumulation of fluids under the very thin eyelids skin. Severe inflammation could affect the septum that keeps the fat pads deep in the orbital cavity, therefore any weakness in the septum would lead to prolapse of fat under the skin and causes the appearance of eye bags or called puffy eyes. The septum naturally gets weaker with age and that is another reason why we could develop eye bags any older age.
Mr Samer HamadaMedical Director of Eye Clinic London and London Cataract CentreLondon, UK

If you could post those pictures from before and some now after your cataract surgery that would be very helpful for us doctors trying to help.If medicine was placed there to control inflammation after surgery it's possible you could have some effect until it wears off.It it remains it should be very easy to address or even improve with a variety of lower blepharoplasty techniques.